Generation of C7H7+ cations with isomerization reactions

- This study uses the MP2 method to study generation of 33 C7H7+ isomers by ionic dissociation of their C7H7Br precursors, and DFT and MP2 methods to probe 16 isomerization reactions of some C7H7+ species in gas phase, Facility of generation of C7H7+ cations does not correlate with their stability.
- The 16 concerted isomerizations involve hydride shifts and CC sigma bond shifts, the former having higher barriers.
- These one-step reactions are combined among themselves to yield 7 multi-step conversions of various C7H7+ isomers to the most stable tropyl cation.
- The more energetically feasible alternatives for these conversions are identified.
- Geometries of transition states for the isomerization reactions are compared with those of the reactants and products in each case and found in general to conform to Hammond's postulate.
- The B3LYP DFT approach compares well with the more rigorous MP2 method in predicting relative stabilities of cations and transition states.

The B3LYP and MP2 methods are used to study isomerization reactions of 15 C7H7+ cations in gas phase, along with an MP2 study of the generation of 34 C7H7+ isomers by ionic dissociation of their C7H7Br precursors. Ionic dissociation is endothermic, where dissociation facility does not correlate well with carbocation stability, though various physicochemical factors facilitating dissociation facility correlate fairly well among themselves. The 15 C7H7+ cations studied for isomerization include the benzyl, o-tolyl, m-tolyl, p-tolyl and 7-norbornadienyl cations, besides hypothetical higher energy species. 16 select concerted isomerizations are considered, of which all but one proceed via well-defined transition states. 1,2 and 1,3 hydride shifts involve larger activation barriers than 1,2 CC sigma bond shifts. By combinations among themselves, these one-step reactions lead to 7 multi-step schemes for conversion of various C7H7+ cations to the most stable tropyl cation isomer. Some conversions involve alternative isomerization routes, of which the energetically more feasible one is identified. Transition states located are described with regard to their relative positions along the reaction coordinate in accordance with the Hammond postulate linking transition state geometry to reaction energetics.

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